Current LSIs have been improved in characteristics by miniaturization. However, in accordance with the progress of the miniaturization, a gate length of transistors has become below 30 nm, which is giving rise to adverse effects of the miniaturization. Therefore, an attempt is being made to achieve the characteristic improvement by using channel materials having a higher mobility instead of conventionally used silicon. As candidates for these materials, compound semiconductors such as germanium and InGaAs are named, but graphene that is a two-dimensional material and has a very high mobility is also drawing attention.
Having a high mobility of 100,000 cm2/Vs or so even at room temperature and being free from difference in mobility between electrons and holes, graphene is expected as a future channel material. However, not having a band gap, the graphene has a small on-off ratio as it is and its use as a switching element is difficult. Therefore, in order to form a band gap, various methods such that graphene is processed into a ribbon shape have been proposed. As one of such methods, a method of vertically applying an electric field to two-layer graphene has been proposed. In this case, a device structure becomes slightly complicated because an electrode is provided on an upper surface and a lower surface of the two-layer graphene. There is also performed an attempt to deposit a molecular material that performs doping of holes and electrons on an upper layer and a lower layer of the two-layer graphene each instead of providing the electrodes, to form a band gap.    Non-Patent Document 1: H. Sugimura et al., Surf. Interf. Anal. 34 (2002) 550.    Non-Patent Document 2: K. S. Novoselov et al., Science 306 (2004) 666.    Non-Patent Document 3: S. Lee et al., Nano Lett. 10 (2010) 4702.
The attempt to deposit a molecular material on and under the two-layer graphene, to form a band gap is advantageous to the device manufacture because of the fact that there is no need to provide electrodes for electric field application in a device structure. However, the present situation is that only the principle verification of band gap formation has been performed so far, and how the transistor structure is made in practice has not been settled yet.